Known steelmaking processes for processing a solid ferrous charge consist of certain generic steps such as: charging ferrous metallic materials, directing heat toward the surface of the metallic pieces, charging non-metallic slag-forming material, superheating and refining the molten pool and discharging the molten metal and slag.
The different steelmaking processes are distinct due to essential differences in the techniques of conducting one or more of these steps. The known steelmaking processes are also distinct due to essential cross-dependence and cross-influences of these steps. In many cases, to maintain the competitiveness of a steelmaking process, each of these steps are carefully optimized with respect to each other, so that an innovation in one process step or parameter may require a substantial alteration of traditional engineering philosophy that has been previously used to design one or more of these basic steelmaking steps.
The processes of metallic scrap melting that are used to produce molten steel are varied based on the source or sources of heat which are used to accomplish the melting. Modern electric arc furnaces are capable of transferring, in a very short time, more than 250 kwh/ton of thermal energy into the scrap to be melted. But the high cost of electricity and low thermal efficiency of these furnaces (less than 50%) continuously motivates the steelmaking industry to develop new steelmaking processes which utilizes less expensive heat from the combustion of fuel to preheat and melt scrap.
For example, U.S. Pat. Nos. 4,622,007 and 4,642,047 teach how to melt steel by using a plurality of burners as an energy source to preheat scrap and then to direct multiple oxidizing flames toward the preheated scrap to melt it down by partial oxidation. This method is utilized now in many electric arc furnaces equipped with auxiliary burners. Today, electric arc furnaces are responsible for processing approximately 70% of ferrous scrap in the United States and other developed countries.
Numerous attempts to develop a new, more advanced steelmaking technology utilizing solid scrap and the heat released by combustion of different fuels (including carbonaceous materials) have been conducted around the world during many decades in order to provide a competitive alternative to electric arc steelmaking. At the same time, the companies involved in the production of primary steel from molten ("hot") iron are motivated to increase the fraction of metallic scrap utilized in the production, because ferrous solid scrap is significantly cheaper than hot iron and can be charged and processed in a basic oxygen furnace by transferring from auxiliary combustion sources additional heat, which is necessary to melt this additional solid charge.
Several methods of producing steel from a solid metallic charge are described in German patents Nos. 2,719,981; 2,729,892; and 2,816,543 and also in international patent application No. PCT/SU83/00025. All of these methods can be carried out in a basic oxygen furnace equipped with bottom and side tuyeres that are used to supply gaseous oxygen as the oxidizing gas and liquid or gaseous hydrocarbons as a fuel.
The shortcomings of these methods originate chiefly from the necessity to supply liquid, gaseous and solid carbonaceous fuels through the oxygen tuyeres. Major deficiencies in this method are: the excessive wear of refractory lining in areas surrounding the tuyeres; the high content of CO in exhaust gases and, therefore, low thermal efficiency; and excessive metallic losses due to substantial exposure of essentially the entire scrap surface to a fraction of unconsumed oxygen being supplied throughout multiple tuyeres. The significant part of oxygen that is not able to react with carbon reacts first with the metallic charge, creating metallic oxides. The fuel supply system, as well as the system for preparation and transport of the fine-grained materials, requires complex additional equipment, which results in an increase in capital and operating expenses.
Moreover, these systems may be less economical due to increased fuel consumption and prolonged heating time.
Any steelmaking process utilizing solid ferrous material and conducted in a high temperature furnace involves several concurrent process steps cross-influencing each other. To be converted from solid to the liquid form ferrous material has to receive a significant amount of heat. This heat should be transferred very rapidly to make the steel process economical. At elevated temperatures (above 900.degree. C.) the oxidation of solid ferrous materials exposed to a gaseous atmosphere containing unconsumed oxygen is accelerated very rapidly, creating solid oxide scale which insulates metallic pieces from heat transfer. Further, when oxides become liquid, they run down together with the iron-carbon melt to the colder bottom of the furnace, influencing the metallic melt and slag chemistry and the heat and mass balance of reactions between carbon and other components of slag and molten metal.
When fuels such as liquid or gaseous hydrocarbons and or carbonaceous solid materials are burned to release the heat needed to melt solid ferrous material, the combustion products that occupy the furnace atmosphere actively react with solid ferrous material. The temperature and chemistry of these combustion products influence the rate of heating and oxidation and, therefore, the dynamics of oxide generation in the scrap pile and the rate of its introduction into the slag.
The timing of slag formation and its chemistry is also influenced by dynamics of the supply of heat, slag forming materials and carbon into the furnace bottom region wherein the slag formations takes place. The carbon content in the slag, the slag temperature, and basicity influence the reactions between the oxides of the slag, the sulfur, phosphorus and silicon in the slag and the iron-carbon melt during the entire steelmaking process.
Existing methods of steelmaking speed scrap melting by placing hot combustion products inside of a scrap pile in such a way that maximum contact between the combustion products and the surface area of scrap is realized to maximize heat transfer. In order to provide contact between the entire surface area of scrap and hot combustion gases, oxygen is fed from many directions, fluid fuel is fed to mix with oxygen by the tuyeres or burners to arrange good mixing, and carbonaceous material is placed inside of the scrap pile by batch charging.
When the maximum surface area is exposed to the oxygen flow supplied to oxidize carbonaceous material, during the low temperature stage of preheating, the metallic surface is not effected. But later, when the scrap surface becomes hot, excessive oxygen contact results in excessive iron oxide production. This excessive iron oxides production later cools the slag by endothermic reaction between the iron oxides and the carbonaceous materials collected in the slag. This also leads to increased oxidation of the iron-carbon melt during melt superheating which reduces metallic yield and process competitiveness. Excessive oxidation of the scrap surface also results in a heat insulating layer of oxides on the scrap surface, which reduces the rate of heat transfer and increases the duration of scrap preheating and melting.
When cold solid materials are charged on the bottom of the furnace that is not provided with local heat input means, they cool down very rapidly this bottom lining. Later, during the melting cycle, a first portion of molten material reaches the colder furnace bottom zone. Contact between this first portion of molten material and the cold bottom lining results in solidification of this first portion mixed with a fraction of the solid charge that has been charged on the bottom of the furnace. These solidified materials stay solid until a later part of the melting cycle and only when the molten metallic pool becomes substantially hot does this solidified bottom layer melt down and begin to participate in the refining reactions. This results in a significant increase in the duration of steelmaking cycle.
Due to the recognition of such negative influence of having a cold bottom zone, the patents referenced above disclose means, such as burners or tuyeres, for providing local heat input to keep the furnace bottom zone hot during steelmaking process. Unfortunately, the introduction of oxidizing gas near the bottom of the furnace for the purpose of combustion of auxiliary fuel triggers the chemical reactions of oxidation of molten materials actively competing for oxygen with said fuel. This negatively affects the efficiency of the entire steelmaking process, including the metallic yield, the rate of slag formation, the length of melting and refining, and the predictability of melt chemistry.
Therefore, in order to provide for high productivity and efficiency of steelmaking processes utilizing solid ferrous metallic material and fuel consisting of hydrocarbons and solid carbon, it is important to protect the solid material from excessive oxidation during entire melting down cycle.
It is also important to provide for hot combustion products at the bottom of the scrap, so that the lower part of scrap pile and the melt itself can be continuously heated,thereby protecting the first portion of the melt from cooling by contacting the colder bottom of the furnace.
At the same time, to reduce the duration of the steelmaking cycle by providing for continuous refining of the melt, it is desirable to provide for continuous desulfurization of the melt by placing it in contact with hot high basicity slag as early as possible.
In order to increase the metallic yield when 100% solid ferrous material is used for steel production, it is important to provide for continuous carburizing of the iron-carbon melt collected at the bottom of the furnace via contact with hot solid carbon.
In order to increase the flexibility of a steelmaking process utilizing solid ferrous metallic material, it is desirable to provide the capability of using not only solid steel scrap, but also solid pig iron, direct reduced iron, a small fraction of back charged molten steel, or a portion of hot iron.